This invention relates to a silver halide photographic element having increased contrast and a method of making same.
Conventional photographic elements contain emulsions of silver halide crystals which, upon exposure to light, form images. Silver halide in its unfinished or primitive state is only moderately sensitive to light in the blue region of the spectrum. Therefore, a silver halide emulsion is generally chemically modified to adjust its properties and optimize it for a particular photographic application. For example, chemical sensitizers may be added to increase the sensitivity (speed) of an emulsion; spectral sensitizing dyes may be added to extend the response of an emulsion to wavelengths where the silver halide does not normally respond; antifoggants may be added to reduce the developability of silver halide crystals in unexposed regions; and latent image keeping agents may be used to preserve the exposed image until it can be processed.
Even with all these chemical modifiers, often referred to as addenda, it is still sometimes impossible to build all of the desired characteristics into a silver halide emulsion for a particular application. This is because such chemical modifiers often interact with each other. For example, spectral sensitizing dyes may extend the intrinsic sensitivity of the emulsion to other wavelengths, but too much dye can cause dye desensitization and loss of speed. Antifoggants are effective at controlling unwanted development, but they may also cause loss of intrinsic speed. This problem is exacerbated in photographic elements having a high silver chloride content such as photosensitive paper used for color prints. Typically, silver chloride emulsions, which have a faster development rate, exhibit increased fogging characteristics and emulsion sensitivity changes owing to high temperatures and/or humidity conditions which may be incurred during storage.
One common problem in optimizing a silver halide emulsion is low contrast. Contrast as measured by gamma is the slope of the straight-line portion of the D-logE curve. Low contrast results in a poor discrimination between the highlights and the shadows of the developed image. Manipulation of chemical addenda may be effective at producing the desired fog level, photographic speed, granularity (a measure of signal-to-noise), and other properties such as pressure sensitivity, but the emulsion will be left with too low a contrast for the intended use. A position is often reached where further adjustment of addenda to increase contrast causes a deficiency in one or more other responses of the photographic element.
Antifoggants such as the tetraazaindenes may increase contrast, but the increase is accompanied by a loss in speed. Nucleating agents are incorporated into photographic vehicles or added to developers to increase contrast by boosting development in lithographic systems, but these agents cause large increases in contrast and degradation in grain and are not useful for optimizing the contrast of an emulsion for a non-lithographic application.
Japanese Kokai JP 62 80,647 describes 1,5,5-trialkylrhodanines as carriers for timed release of antifoggants. U.S. Pat. No. 4,808,516 describes the use of unsubstituted rhodanine in combination with a sensitizing dye to improve the contrast and speed of emulsions. U.S. Pat. No. 4,272,606 describes a method of forming high contrast images by developing a photographic element in the presence of a compound having a thioamido moiety in the molecular structure thereof; one such compound being a 5-substituted rhodanine. U.S. Pat. No. 4,906,558 describes ripening a silver chloride emulsion with a 5,5-disubstituted rhodanine gold compound. U.S. Pat. No. 2,278,947 describes sensitizing a silver halide emulsion with an N-substituted rhodanine compound. However, none of these rhodanine compounds provides the combination of speed and contrast desired for today's photographic elements.
The need exists for a chemical addenda for silver halide emulsions which increases the contrast of a developed image after exposure without affecting the other desirable properties of the emulsion. More specifically, the need exists for an addenda which can be added to an emulsion to increase the contrast of the developed image without causing a loss in photographic speed or an increase in fog level.